1. Field of the Invention
The present invention relates to an apparatus and a method for measuring optical properties of a sample material, such as an optical surface property, and more particularly glossiness, of the sample material.
2. Description of the Related Art
Optical property measuring apparatuses for measuring optical surface properties of a sample surface, such as glossiness thereof, are conventionally known.
FIG. 9 is a diagram showing the optical configuration of a conventional optical property measuring apparatus, FIG. 10 is a plan view showing the structure of a projection-side aperture plate of the optical property measuring apparatus, FIG. 11 is a plan view showing the structure of a sensing-side aperture plate of the conventional optical property measuring apparatus, and FIG. 12 is a diagram for explaining image focusing positions of a projection-side aperture formed in the projection-side aperture plate.
Referring to FIGS. 9 to 11, the conventional optical property measuring apparatus 100 comprises a projection-side optical system 110 and a sensing-side optical system 120. The projection-side optical system 110 is configured such that an optical axis 113a makes a specific angle θ3 with a normal G to a surface SMa of a sample SM, the normal G passing a particular point on the sample surface SMa, whereas the sensing-side optical system 120 is configured such that an optical axis 123a makes a specific angle θ4 with the normal G.
The projection-side optical system 110 includes a light source 111, the aforementioned projection-side aperture plate 112 and an illumination lens 113. As shown in FIG. 10, the projection-side aperture plate 112 has the aforementioned projection-side aperture 112a formed therethrough. The projection-side aperture 112a has a rectangular shape having a width w of 0.75 degrees (as measured generally in a vertical direction as illustrated in FIG. 9) and a height h of 2.5 degrees (as measured in a direction perpendicular to the plane of paper as illustrated in FIG. 9) in terms of view angle.
On the other hand, the sensing-side optical system 120 includes an optical sensor 121, the aforementioned sensing-side aperture plate 122 and a light-receiving lens 123. As shown in FIG. 11, the sensing-side aperture plate 122 has a sensing-side aperture 122a formed therethrough. The sensing-side aperture 122a has a rectangular shape having a width W of 4.4 degrees (as measured generally in the vertical direction as illustrated in FIG. 9) and a height H of 11.7 degrees (as measured in the direction perpendicular to the plane of paper as illustrated in FIG. 9) in terms of view angle.
A traveling path of light emitted by the light source 111 is confined by the projection-side aperture 112a formed in the projection-side aperture plate 112 to a specific spreading angle and the illumination lens 113 produces a parallel light beam (a beam of parallel light rays) 111a which is generally parallel to the optical axis 113a. The parallel light beam 111a thus produced illuminates the sample surface SMa and incident light is reflected by the sample surface SMa. Part 121a of light reflected generally in a direction of regular reflection is converged by the light-receiving lens 123 and received by the optical sensor 121 after passing through the sensing-side aperture 122a formed in the sensing-side aperture plate 122. Then, the optical property measuring apparatus 100 determines the value of an optical property of the sample surface SMa, such as glossiness thereof, based on an output of the optical sensor 121.
In this optical property measuring apparatus 100, it is required that the sample SM be placed in a “normal” position in order that the sample SM is so disposed relative to the projection-side optical system 110 and the sensing-side optical system 120 as to produce angles of incidence and reflection equal to the aforementioned specific angles θ3 and θ4. However, if the sample SM inclines by angle δ, for example, an image focusing position M′ on an image focusing plane is displaced by as much as f·tan 2δ from an image focusing position M obtained when the sample SM is in the normal position, where f is the focal length of the light-receiving lens 123. When the sample SM having a specular surface is in the normal position, an image of the projection-side aperture 112a passes through approximately a central part of the sensing-side aperture 122a as shown by broken lines 112b in FIG. 11 and properly falls upon the optical sensor 121. If the sample SM deviates from the normal position, however, the image of the projection-side aperture 112a will be displaced from the sensing-side aperture 122a and obstructed by the sensing-side aperture plate 122 in part or totally as shown by broken lines 112c in FIG. 11 so that the image will not properly falls upon the optical sensor 121. In this case, the optical sensor 121 can not receive an amount of light that is expected to be obtainable under normal conditions and, as a consequence, values of the measured optical property contain some errors. Also, if the sample SM does not have a specular surface, the reflected light is scattered so that the measured optical property values contain errors in this case as well.
This kind of inconvenience occurs not only when the sample SM inclines from the normal position, causing a position error thereof, but also when the surface SMa of the sample SM is not a flat surface.
Under such circumstances, Japanese Unexamined Patent Publication No. 2006-208361 proposes an arrangement for overcoming the aforementioned problem of the prior art. Specifically, the arrangement of this Publication is to eliminate the aforementioned sensing-side aperture plate 122 and employ instead of the optical sensor 121 an image pickup device having a large light-sensing surface provided with a relatively large number of photosensitive elements (pixels) made of charge-coupled devices (CCDs), for example. In this arrangement, a pixel area corresponding to the aforementioned sensing-side aperture 122a is defined on the light-sensing surface of the image pickup device at a location thereof on which light reflected by a sample is incident in order to solve the problem of the prior art.
For measuring optical properties with high accuracy, however, a charge-transfer image pickup device provided with a relatively large number of photosensitive elements like CCDs does not provide satisfactory basic performance with respect to signal-to-noise ratio (S/N ratio) of an output, linearity and temperature characteristics of the device, for instance.
A photodiode array, on the other hand, has superb basic performance to offer improved measuring accuracy. There are however limitations in circuit configuration and cost-effectiveness of an arrangement employing a photodiode array for directly taking out outputs from individual photosensitive elements (photodiodes). It is therefore impossible to use a photodiode array provided with a relatively large number of photosensitive elements like the CCD-based image pickup device.